This study will compare the durability of viral load response following 48 weeks treatment with 1592U89/ZDV/3TC vs. ZDV/3TC, assess the safety and tolerance of these combinations. It will also compare the clinical efficacy of the two regimens as determined by survival, disease progression, weight velocity and neuropsychological or neurological changes, assess the viral resistance in a subset of subjects, characterize the steady-state plasma levels of 1592U89 across the subjects between 3 months to 13 years of age, and evaluate the changes in drug pharmacokinetics.